U.S. patent application number 10/766973 was filed with the patent office on 2004-09-23 for helix coupled remote plasma source.
Invention is credited to Kamarehi, Mohammad.
Application Number | 20040182834 10/766973 |
Document ID | / |
Family ID | 32825413 |
Filed Date | 2004-09-23 |
United States Patent
Application |
20040182834 |
Kind Code |
A1 |
Kamarehi, Mohammad |
September 23, 2004 |
Helix coupled remote plasma source
Abstract
A remote plasma source employs a helical coil slow wave
structure to couple microwave energy to a flowing gas to produce
plasma for downstream substrate processing, such as photoresist
stripping, ashing, or etching. The system also includes cooling
structures for removing excess heat from the plasma source
components.
Inventors: |
Kamarehi, Mohammad;
(Gaithersburg, MD) |
Correspondence
Address: |
KNOBBE MARTENS OLSON & BEAR LLP
2040 MAIN STREET
FOURTEENTH FLOOR
IRVINE
CA
92614
US
|
Family ID: |
32825413 |
Appl. No.: |
10/766973 |
Filed: |
January 29, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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60444612 |
Jan 30, 2003 |
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Current U.S.
Class: |
219/121.43 |
Current CPC
Class: |
H01J 37/32192 20130101;
H01J 37/32357 20130101; H01J 37/32211 20130101 |
Class at
Publication: |
219/121.43 |
International
Class: |
B23K 010/00 |
Claims
I claim:
1. A remote plasma generator, comprising: a microwave power source;
a microwave energy waveguide communicating with the power source,
and configured to produce a travelling wave; a plasma chamber
configured to be mounted in fluid communication between a source of
gases and a processing chamber; a helical coil surrounding the
plasma chamber; and a coupler coupling the microwave energy from
the waveguide to the helical coil.
2. The remote plasma generator of claim 1, wherein the helical coil
is formed of a hollow metal tubing.
3. The remote plasma generator of claim 2, further comprising a
pump circulating cooling fluid through the tubing.
4. The remote plasma generator of claim 1, further comprising a
cooling jacket surrounding the plasma chamber.
5. The remote plasma generator of claim 4, wherein the helical coil
surrounds the cooling jacket.
6. The remote plasma generator of claim 1, wherein the microwave
power source generates power at 2450 MHz.+-.50 MHz.
7. The remote plasma generator of claim 1, wherein the helical coil
is configured to couple a travelling wave to gas travelling through
the plasma chamber.
8. The remote plasma generator of claim 1, wherein the plasma
chamber is positioned upstream of a photoresist asher chamber.
9. The remote plasma generator of claim 1, further comprising a
microwave shield surrounding the plasma chamber and the helical
coil.
10. The remote plasma generator of claim 1, further comprising a
microwave absorption load at an output end of the helical coil.
11. The remote plasma generator of claim 1, wherein the coupler
comprises a section of coaxial cable.
12. A method of delivering a plasma to a processing chamber, the
method comprising: providing a source of process gases in fluid
communication with a plasma tube, said plasma tube being in fluid
communication with a process chamber; generating a travelling
microwave signal; propagating the travelling microwave signal along
a microwave conducting structure having a section with a helical
shape surrounding the plasma tube; flowing a gas through the plasma
tube such that a plasma is ignited in the gas; and directing
products of the plasma into the process chamber.
13. The method of claim 12, further comprising providing a cooling
jacket between the helical portion of the microwave conducting
structure and the plasma tube and flowing a cooling fluid through
the cooling jacket.
14. The method of claim 12, further comprising tuning the microwave
signal to a desired wavelength prior to propagating the signal
along the microwave conducting structure.
15. The method of claim 12, wherein generating a microwave signal
comprises generating a signal with a frequency of about 2450
MHz.+-.50 MHz.
16. The method of claim 12, wherein generating a microwave signal
comprises generating a signal with a power of between about 1300 W
and about 1500 W.
17. The method of claim 12, wherein the microwave conducting
structure comprises a hollow-centered tube, and further comprising
pumping a cooling fluid through the microwave conducting
structure.
18. The method of claim 17, further comprising isolating the
cooling fluid from the microwave signal.
19. The method of claim 12, further comprising a microwave shield
surrounding the helical coil and the plasma tube.
20. The method of claim 19, further comprising flowing a cooling
gas through a space between the microwave shield and the plasma
tube.
21. A method of removing a layer from a substrate, the method
comprising: flowing plasma source gases through a plasma reactor
tube; propagating microwave energy in a travelling wave along a
microwave conducting structure having a shape of a slow wave
structure and surrounding the plasma reactor tube; igniting a
plasma within the plasma reactor tube; and flowing plasma products
into a process chamber to impinge on a substrate to remove a mask
layer on the substrate.
22. The method of claim 21, further comprising flowing a cooling
fluid through a conduit adjacent the plasma reactor tube.
23. The method of claim 22, wherein the conduit is a space between
a microwave shield and a plasma tube.
24. The method of claim 22, wherein the conduit is a passage
through a hollow-centered material of the slow wave structure.
25. The method of claim 22, wherein the conduit is a cooling jacket
concentrically surrounding the plasma tube.
26. The method of claim 21, wherein the substrate is a
semiconductor wafer.
27. The method of claim 26, wherein the wafer is silicon.
28. The method of claim 27, wherein the layer is a photoresist
material.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This Application claims the benefit of U.S. Provisional
Patent Application Serial No. 60/444,612, filed Jan. 30, 2003, the
entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to remote plasma sources for
use in semiconductor manufacturing processes such as photoresist
ashing and etching processes.
[0004] 2. Description of the Related Art
[0005] In semiconductor processing, remote plasma generators are
often employed upstream of a processing chamber such as an asher
employed for burning off photoresist after lithography steps. Such
remote plasma generators have been provided in a variety of
configurations. In general, energy is coupled to a gas flowing
through a plasma tube in order to create a plasma. Plasma products
continue to flow downstream through the plasma tube, into a process
chamber, and ultimately impinge on a workpiece.
[0006] One common method of coupling energy to a flowing gas is
known as inductively-coupled-plasma (ICP) generation. An ICP
reactor typically couples electrical energy to a flowing gas by
induction (and to some extent by capacitance) from an electrical
conductor, often in the shape of a coil, surrounding a plasma
reactor chamber. The electromagnetic field created within the
chamber by the electric current flowing through the conductor is
repeatedly reversed by alternating the electric current at high
frequencies, thereby coupling substantial energy to the gas
molecules flowing through the chamber. See, e.g., U.S. Pat. No.
5,964,949.
[0007] Another common method of coupling energy to a flowing gas,
which is more complex but generally less expensive, is by applying
microwave energy to a flowing gas. One such configuration employs a
magnetron generator that propagates microwave energy along a
waveguide. In this configuration, the waveguide is arranged to
create a standing wave which peaks in a portion of a gas tube
passing through the waveguide. Coupling sufficient microwave energy
to the gas causes a plasma to ignite. For this reason, the portion
of the tube in which energy is coupled to the flowing gas is often
referred to as a plasma applicator.
[0008] One of the difficulties with such conventional plasma
generators is that the standing wave is established and maintained
by tuning, such as by gross tuning at the end of the waveguide and
fine-tuning dynamically with one or more tuning stubs within the
waveguide. However, the mass flow rate of the plasma and the amount
of energy coupled to the plasma can change during processing, and
from recipe to recipe. Such changes make it difficult to maintain
the standing wave and therefore the transfer efficiency of energy
to the plasma suffers, resulting in increased energy costs and
inconsistent processing effect. Therefore, it is desirable to
provide a microwave plasma generator that does not suffer from the
difficulties associated with tuning a standing wave.
SUMMARY
[0009] The need for dynamic tuning of a microwave signal during
processing can be substantially reduced or eliminated by placing
the plasma tube in the path of a travelling wave rather than a
standing wave. One category of travelling wave structures includes
"slow-wave" structures, which have particular advantages in
applications where it is desirable to reduce the phase velocity of
a microwave in a particular direction.
[0010] Slow-wave structures or periodic structures are microwave
devices that are commonly used in conventional vacuum tubes, such
as traveling-wave and magnetron-type tubes, to reduce microwave
signal velocity in the direction of wave propagation so that the
gas travelling in the space and the signal wave can interact. The
phase velocity of a propagating electromagnetic wave (i.e.
transverse electromagnetic or "TEM") in ordinary microwave
waveguides is greater than the velocity of light. The phase
velocity of a propagating TEM wave in free space or in a vacuum is
less than the velocity of light. In the operation of traveling-wave
tube ("TWT") and magnetron-type devices which are used to amplify
and oscillate small signals to much stronger signals, the velocity
of the electron cloud must be comparable to the phase velocity of
the microwave signal for an effective transfer of the electron
energy to the microwave signal. Since the electron velocity can be
accelerated only to velocities that are slightly slower than the
speed of light, a slow-wave structure is typically incorporated
into the TWT and magnetron-type devices to bring the phase velocity
of the microwave signal close to the velocity of the electron beam
for effective interaction.
[0011] A wire wound in the form of helix is one type of periodic
slow-wave structure and is commonly used as a slow-wave device in
the construction of traveling wave tubes (see, e.g., Microwave
Devices and Circuits, Samuel Y. Liao, page 477, FIG. 9.20, O-type
traveling-wave tube). The phase velocity of a wave travelling
through the helix, .nu..sub.P, is approximately given by C Sin
.PSI., where C is the velocity of light and .PSI. is the pitch
angle of the helix. Since the value of Sin .PSI. is less than 1,
the phase velocity of the microwave signal propagating along the
axial direction of the helix is less than the speed of light, thus
making the microwave signal suitable for interaction with the
electron beam generated in the TWT by DC electrodes.
[0012] In a conventional vacuum tube, a helical structure can be
incorporated in an electron beam in direct contact with the plasma
flux. The injected signal traveling on the helix towards the output
typically extracts energy from the electron beam and is amplified
as it exits the helix. In the preferred embodiments described
below, this principle is applied in reverse, i.e., the high-power
microwave energy is delivered to the helix and is then transferred
to the gases in the plasma vessel as the microwave energy
propagates along the helical structure. Thus, as sufficient
microwave energy is transferred to the gases, a plasma is created
in the plasma vessel.
[0013] According to some embodiments, a helical slow-wave structure
can be used to launch and deliver microwave energy as a propagating
wave, preferably at 2,450+/-50 MHz, to generate plasma for
downstream photoresist removal and etch applications.
[0014] In one embodiment, a remote plasma generator is provided,
comprising a microwave power source, a microwave energy waveguide
communicating with the power source, and a plasma chamber
configured to be mounted in fluid communication with a source of
the gases and a processing chamber. A helical coil surrounds the
plasma chamber, and a waveguide coupler couples the microwave
energy from the waveguide to the helical coil.
[0015] According to another embodiment, a method of delivering
products of a plasma to a processing chamber is provided. The
method comprises providing a source of process gases in fluid
communication with a plasma tube which is in fluid communication
with the process chamber. A microwave signal is then generated and
propagated along a microwave conducting structure having a portion
in the shape of a helix surrounding the plasma tube. A gas is then
flowed through the plasma tube such that a plasma is ignited in the
gas within the plasma tube, and the plasma products are ultimately
delivered to the process chamber.
[0016] Still another embodiment provides a method of removing a
layer from a substrate. The method comprises flowing plasma source
gases through a plasma reactor tube, propagating microwave energy
in a travelling wave along a microwave conducting structure having
a shape of a slow wave structure and surrounding the plasma reactor
tube. A plasma is ignited within the plasma reactor tube by the
microwave energy, and the plasma products continue to flow to a
process chamber where they impinge on a substrate in order to
remove the layer from the substrate. In further embodiments, the
reactor tube can be cooled by flowing a fluid through a conduit
adjacent the reactor tube. The conduit can comprise a hollow center
of the slow wave structure material, a cooling jacket, or a space
between a microwave shield and the reactor tube.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a schematic illustration of one embodiment of a
plasma generator having features in accordance with the present
invention;
[0018] FIG. 2 is a schematic representation of another embodiment
of a plasma generator having features in accordance with the
present invention;
[0019] FIG. 3 is a schematic, partially cut-away perspective
illustration of an embodiment of a plasma generator having features
in accordance with the present invention;
[0020] FIG. 4 is a schematic illustration of a cross-bar
termination of the plasma generator of FIGS. 1 and 2;
[0021] FIG. 5 is a schematic perspective illustration of an
embodiment of a plasma generator that is configured to be
air-cooled; and
[0022] FIG. 6 is schematic perspective illustration of an
embodiment of a plasma generator employing coaxial cable
connections.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0023] The embodiments below generally describe remote plasma
sources for use in semiconductor processing including ashing,
etching and stripping steps. The remote plasma sources described
herein generally employ a slow wave structure in the form of a
helical coil to couple microwave energy to a gas flowing through a
plasma tube.
[0024] FIGS. 1 and 2 schematically illustrate embodiments of a
plasma source (or plasma generator) 10 having features of the
present invention. In one embodiment, the plasma source 10
generally comprises microwave generation and tuning structures 12,
a source of gases 14, a plasma tube 20 and a microwave absorption
load 22. As shown, the plasma tube 20 is surrounded by a microwave
conducting structure 24 with a central portion formed in the shape
of a helical coil 26. In one preferred embodiment, the helical coil
26 and plasma tube 20 are enclosed within a microwave shield 30
which can be made up of a metallic sheet or mesh configured to be
opaque to microwave energy. Some embodiments also include a cooling
jacket 32 between the helical coil 26 and the plasma tube 20. The
cooling jacket 32 is generally configured to remove excess heat
from the system components.
[0025] Additionally, the system can include a microwave trap
located between the coil 26 and the process chamber in order to
limit the transmission of microwave energy into the reaction
chamber. Many embodiments of such traps are available, for example,
U.S. Pat. No. 5,498,308 to Kamarehi, entitled "Plasma Asher with
Microwave Trap," describes a resonant circuit trap.
[0026] The microwave source can comprise any generator recognized
as suitable for creating microwave energy of the desired power
level, such as a magnetron or a travelling wave tube type
generator. In some embodiments, it may be desirable to provide
microwave tuning structures. In the plasma generator embodiments
illustrated schematically in FIGS. 1 & 2, sliding shorts 34 and
tuning stubs 36 are provided to vary the wavelength of the
microwave signal in order to optimize the coupling of the microwave
power to the microwave conducting structure 24. In further
embodiments, other tuning structures can also be used, such as a
multiple stub tuner, and in still further embodiments the apparatus
can be optimized during design so as to avoid dynamically tuning
the microwave signal during processing. In contrast to conventional
plasma generators, a standing wave is not maintained in the
waveguide 40 of the illustrated system, such that dynamic tuning is
not as critical as it is for conventional microwave plasma systems.
Additionally, because the plasma tube 20 does not pass through the
microwave waveguide 40, the plasma generator is relatively
insensitive to changes in the process recipe, such as changes in
the flow rate of gas/plasma through the process tube 20.
[0027] The microwave conducting structure 24 is typically a
metallic material selected to direct the microwave energy along its
length. In one embodiment, the microwave conducting structure 24
comprises a section of copper tubing formed in the shape of a helix
26. The helix 26 can be formed such that its longitudinal axis is
concentric with the cylindrical plasma tube 20. In alternative
embodiments, the helix can be formed such that its center is not
coaxial with tube 20. Additionally, the microwave conducting
structure can be solid or can comprise other materials such as
aluminum, steel, or other metals or composites capable of
propagating an electromagnetic wave in the microwave region. The
microwave conducting structure 24 can also comprise a material of
any suitable solid or hollow-centered cross-sectional shape, such
as circular, rectangular, elliptical, etc. As the microwave current
flows along the helical wire 26, an electromagnetic field is
created within the plasma tube 20 to couple energy to the gases
flowing through the tube 20.
[0028] In some preferred embodiments, the helical section 26 of the
microwave conducting structure 24 preferably has a length `L`
measured along the plasma tube axis of between about 0.5 and 10
inches. The length `L` of the helix 26 can be varied depending on
the level of microwave powers to be coupled to the flowing gases.
For lower applied microwave power, a shorter helix can be used,
while a higher applied microwave power typically calls for a longer
helix in order for an the same proportion of energy to be absorbed
by the gas in the plasma tube. Similarly, the internal diameter of
the helix 26 and the material cross-section can be varied according
to factors such as the amount of power to be transmitted. In one
preferred embodiment, the helix 26 has a length `L` of about 1.6
inches, an inside coil diameter of about 1 inch, and an outside
coil diameter of about 1.25 inches (i.e., the conducting material
is about 0.25" thick). In one preferred embodiment, the helix
includes 7 turns such that each turn is spaced an axial distance of
about 0.23 inches from each adjacent turn. The microwave conducting
structure material can be in direct physical contact with the outer
surface of the cooling jacket 32 or the plasma tube 20 if so
desired.
[0029] The microwave conducting structure 24 is coupled to the
microwave waveguide 40 by a waveguide antenna coupler 42.
Alternatively (or in addition), in some embodiments such as low
power applications (e.g., less than about 1,000 Watts), coaxial
cables and connectors 44, as shown in FIGS. 2 and 6, can be used as
part of the waveguide couplers 42 to the microwave conducting
structure 24. Other methods of coupling the microwave energy to the
helical coil 26 can also be used.
[0030] Since the plasma generator preferably employs a travelling
wave, the microwave conducting structure 24 can be described as
having an input end 46 and an output end 48. At the output end 48,
the microwave conducting structure 24 can be coupled to a microwave
load 22 configured to absorb the remaining microwave energy leaving
the plasma source 10. The microwave conducting structure 24 can be
coupled to the microwave load 22 by a waveguide coupler 42 (FIGS.
1, 3 and 5), coaxial cable transitions 44 (FIGS. 2 and 6), or other
suitable microwave coupling devices.
[0031] The microwave load 22 can include any structure suitable for
dissipating or re-using the microwave energy leaving the coiled
portion 26 of the microwave conducting structure 24. For example,
in one embodiment, the microwave absorption load comprises a
water-cooled ceramic structure. Alternatively, some excess
microwave energy can be fed back to the microwave generation and
amplification structures to be returned to the plasma generator
10.
[0032] The plasma tube 20 can comprise a number of suitable
materials, but is typically quartz, ceramic or sapphire.
Embodiments of plasma generators 10 described herein can include
plasma tubes 20 with total lengths between about a half an inche
(i.e. about 12 mm) and about 10 inches (i.e. about 254 mm). The
tube length can be varied according to the desired process
parameters, such as the length of the coil, the quantity of power
to be applied, or the desired process performance. In one preferred
embodiment, the plasma tube 20 has a length of about 7 inches. Of
course, in further embodiments, dimensions outside of these ranges
could also be used as desired.
[0033] In order to control the temperature of the plasma tube 20
and other system components, a cooling jacket 32 can be provided to
enclose the sections of the plasma tube 20 which will be exposed to
the high-temperature plasma products. The cooling jacket 32
typically comprises an annular space through which a cooling fluid
can circulate between the cooling jacket wall and the plasma tube
20. As shown in FIGS. 3, 5 and 6, the cooling jacket 32 generally
includes a fluid inlet 50 connected to a fluid source (not shown)
and a fluid outlet 52 in communication with a heat exchanger (not
shown) for dissipating the heat absorbed by the cooling fluid. The
cooling fluid can be moved through the cooling jacket 32 and the
other cooling system components in an open or closed loop by any
suitable pump (not shown) as will be clear to the skilled artisan.
The cooling jacket 32 can be arranged in a "counter flow"
arrangement wherein a cooling liquid is circulated through the
cooling jacket 32 with a flow in a direction that is generally
opposite to the flow direction of the hot plasma. Alternatively,
the cooling fluid can be circulated through the cooling jacket 32
in a "parallel flow" or other arrangement as desired.
[0034] The cooling jacket 32 is preferably made of a material that
is substantially transparent to microwave energy. According to one
embodiment, the cooling jacket 32 can be made of the same material
or a similar material to the plasma tube 20 (e.g. quartz, ceramic
or sapphire). In such embodiments, the cooling fluid can be
selected to be substantially transparent to microwave energy,
thereby limiting the degree to which the microwave energy heats up
the cooling fluid directly.
[0035] In some embodiments, such as some high power applications, a
cooling jacket, such as those described above, can be supplemented
or replaced by a microwave conducting structure 24 formed of hollow
tubing through which a cooling fluid can be circulated. Since the
microwave energy travels on the outer surface of the microwave
conducting structure 24 and only travels into the microwave
conducting material to a depth on the order of 10.sup.-4 inches
(the skin depth), a fluid travelling through the microwave
conducting structure 24 will be substantially unaffected by the
microwave radiation.
[0036] As illustrated in FIG. 4, a modified cross-bar termination
60 can be provided adjacent the junctions between the ends of the
microwave conducting structure 24 and the waveguide couplers 42.
The modified cross-bar termination 60 is configured to couple the
energy signals from the waveguide coupler 42 to the microwave
conducting structure 24 (and vice versa) and to couple the internal
cooling channel to the cooling fluid flow path 62 while preventing
the cooling fluid from being directly exposed to the microwave
energy signals. This is generally accomplished by providing the
cross-bar termination 60 with a solid microwave conductor 64
exiting the termination in one direction and a fluid outlet 66
exiting the termination 60 in the opposite direction. The fluid
outlet 66 can comprise a non-conductive section 68 in order to
prevent the microwave energy from propagating along the fluid flow
path 62.
[0037] In another embodiment as illustrated in FIG. 5, for example,
the plasma source 10 can be air-cooled by circulating air (or
another cooling gas) through the space 70 between the microwave
shield 30 and the plasma tube 20. According to such an embodiment,
the cooling jacket 32 (FIGS. 1, 3 and 6) could be omitted if so
desired.
[0038] The flow rate of fluid through cooling system (i.e., the
cooling jacket, the hollow microwave conducting material, or the
air space within the microwave shield) will generally depend on
numerous factors such as the rate of heat transfer desired, the
size of the fluid pathway, the contact area between the components
of the cooling system, and others. The skilled artisan will
understand how to appropriately configure the cooling system in
order to keep the system components within acceptable temperature
ranges.
[0039] With continued reference to FIGS. 1-6, the methods of
operation of a plasma generator 10 having features and advantages
of the present invention will now be described. According to one
embodiment, microwave energy produced by a magnetron assembly (or
other suitable microwave generation device) is fed to a waveguide
40 and then propagated along the microwave conducting structure 24,
including the helical coil section 26 which surrounds the cooling
jacket 32 (if present) and the plasma tube 20. As the microwave
energy propagates along the helical coil 26, substantial amounts of
energy are transferred and coupled to gases flowing from the gas
source 14 through the plasma tube 20. Energy is coupled to the
flowing gases by direct excitation of the gas molecules by the
microwave signal propagating along the helix and the plasma tube
space. The plasma products continue to flow through the plasma tube
20 and into the process chamber 80 for processing a workpiece
supported on a substrate support 82 therein. Baffle plates 84 can
also be provided to shield the workpiece from the ultraviolet
radiation discharged from the plasma source and to uniformly
distribute the plasma products throughout the process chamber. The
remaining microwave energy continues to propagate along the coil 26
and is absorbed by the microwave load 22.
[0040] In some embodiments of the present invention, a helical
slow-wave structure is used to launch and deliver microwave energy
as a propagating wave, preferably at 2,450+/-50 MHz and a
wavelength of about 4.8 inches, in order to generate plasma for
downstream applications such as photoresist removal and etching. In
this frequency band, the helix can provide many features and
benefits for these applications. Since the helix supports
propagation of the microwave energy, evenly distributed plasma is
formed in the plasma tube. In some embodiments, a plasma source 10
with a helical slow wave structure can be operated with an applied
power of up to about 3 kW. In one preferred embodiment, the source
is operated with a microwave signal having a power of about 1300 to
1500 W.
[0041] Since the helix is a traveling wave structure as opposed to
a resonant one, the microwave coupling to the plasma gas is not as
sensitive or dependent upon the changes that are made to the state
of the plasma load condition. This eliminates the need for in situ
or manual impedance tuning for maximum power delivery and coupling,
which is a significant and important advantage for photoresist
removal and etch applications, where process parameters such as
pressure, flow rate, power and gas type are varied to optimize
process conditions.
[0042] Evenly distributed plasmas help to prevent a sapphire tube
from cracking. Sapphire tubes, which are often used as plasma
vessels in applications which call for fluorinated chemistries, are
susceptible to failure when exposed to uneven thermal distribution
and thermal shock. Thus, another advantage of the helical source is
the distributed and uniform pattern which reduces local thermal
gradients and simplifies cooling.
[0043] In an experiment, a plasma source including a quartz plasma
tube, a helix having 7 turns, a 1-inch inner diameter and a length
`L` (FIG. 1) of about 5" was assembled. The helix was coupled to a
microwave source using high power coaxial cable couplings, and a
microwave signal at 1,300 Watts and 2,450 Hz, was propagated along
the helix. O.sub.2 and N.sub.2 were flowed through the plasma tube
at 6,000 sccm and into a process chamber maintained at 1.5 Torr in
which a wafer with a photoresist layer was supported. The process
resulted in an ash rate of 5.9 .mu.m/min. This experiment
successfully demonstrated the operability of creating a plasma by
coupling microwave power to a gas using a slow wave structure such
as a helical coil.
[0044] Although certain embodiments and examples have been
described herein, it will be understood by those skilled in the art
that many aspects of the methods and devices shown and described in
the present disclosure may be differently combined and/or modified
to form still further embodiments. For example, in further
embodiments, other slow-wave structures can be used in place of a
helical coil. Additionally, it will be recognized that the methods
described herein may be practiced using any device suitable for
performing the recited steps. Such alternative embodiments and/or
uses of the methods and devices described above and obvious
modifications and equivalents thereof are intended to be within the
scope of the present disclosure. Thus, it is intended that the
scope of the present invention should not be limited by the
particular embodiments described above, but should be determined
only by a fair reading of the claims that follow.
* * * * *